Tap the brown box icon in the lower right corner to go to Collections.   
Here you will find a list of all collections and names of collectors. Collectibles can be found in Cardboard boxes.

Once you have found some or all of the items in the collection, you can send them to the collector. Tap collectibles, pack and send them. In the send window, you will see the name of the collector, the number of coins you will get for sending, the send timer and a list of the collection items to be sent. Tap Send to exchange collectibles for coins.

Buy decorations and interior items with coins to make your characters' houses look cozier and more beautiful. The more collections you send, the more coins you will receive.